Australia’s Dengue Risk: Human Adaptation to Climate Change - Recent Advances and Issues in Environmental Science - page 141

Only the highly arid central Australian region was excluded from the projection
(annual rainfall less than 300 mm). The GARP model did not show southern cold cli-
mate thermal limits in Australia, probably due to the presence of several Ae. aegypti
collection sites from inland NSW that show cool climate parameters. We then mapped
two theoretical cool climate limits across Australia--the 10°C winter (July) isotherm
[5] and the 15°C annual mean isotherm [6]. Of these two isotherm limits the 15°C
annual mean isotherm appeared more representative of the known distribution of Ae.
aegypti in Australia, although collection sites did exist outside these temperature iso-
therm limits.
It remains unknown if the cold climate tolerant populations were breeding in the
warmer months and surviving the colder winter months as eggs [29], or were surviving
as larvae. With regard to these questions, observations have been recorded of viable
Ae. aegypti larvae in ice encrusted water [5, 7], while experiments have suggested
that a water temperature of 1.0°C can be lethal over 24 hr, but larvae can be viable
at a constant 7.0°C for over a week [5]. At the other temperature extreme, laboratory
experiments show that Ae. aegypti larvae perish when the water temperature exceeds
34°C while adults start to die off as the air temperature exceeds 40°C [5]. Domestic
water tanks in Australia contain thousands of liters of water that would--in combina-
tion with the mosquitoes' domestic (indoor) nature--provide a buffer to temperature
extremes and assist mosquito survival in what may appear unsuitable environments.
For example, Ae. aegypti exists and transmit dengue in India's Thar desert townships
in north-western Rajasthan, where the mosquito utilizes household pitchers and under-
ground cement water tanks. [30].
The incongruence between the temperature limits and our ecological niche models
highlights the diffi culties of using what are essentially sophisticated climate pattern
matching procedures to study an organism with a biology and ecology strongly in-
fl uenced by human activity. Fortunately, we can directly compare our GARP model
with a new mechanistic model of the same organism over the same environment [31].
This mechanistic model utilizes biophysical life processes parameters such as the ef-
fects of climate on reproduction and larval development. Larval development in both
rainwater tanks and smaller containers were assessed and the potential distribution of
Ae. aegypti was projected across Australia. Projections using rainwater tanks larval
development resembled our GARP model for Northern and central Australia, but un-
like our projections, a southern cold climate thermal limit was identifi ed which was
actually lower than the published parameters displayed in Figure 3 [5, 6]. Apart from
showing the clear advantage of a bottom-up approach for modeling this mosquito, this
study supports the hypothesis that domestic rainwater tanks contributed for the histori-
cal southern distribution of Ae. aegypti in Australia.
Humans not only facilitate long distance dispersal events for this mosquito, co-
habitation with humans can provide thermal buffers to the outdoor climate as adults
rest indoors, and domestic rainwater tanks can provide stable oviposition sites. When
the theoretical distributions (GARP models and temperature limits) and actual Ae. aegypti
distributions are viewed alongside the expansion of domestic water tanks underway
in Australia, a trend emerges where Ae. aegypti could potentially exist year-round in